Storm Water Ponds

Sediment loading from its watershed is considered the principal challenge to water quality within the
Brandywine. The desire to control bank erosion and sediment runoff has generated huge amounts of good
intention, endless frustration and considerable concern about legal requirements for sediment reduction
in many townships located within the Brandywine drainage. Sediment runoff is exacerbated by urbanization,
increases in the proportion of “impervious” land cover (e.g., roads, driveways, rooftops), and
increasingly frequent soil disturbance associated with new home, business and road construction
(whereas the very large amounts of sediment runoff associated with colonial farming practices in the
watershed have declined more recently).

Fig. 12a–b. (a) Dry basin in West Whiteland Township, showing two smaller inflow structures
and a single, larger outflow in the foreground. (b) Stormwater pond in Newlin Township, with cement inflow
at the far end. Note the elongate shapes of both basins. Mowed grass on steep slopes and the absence of
other vegetation are typical.

One method of sediment control is the construction of swales, intended in part to trap sediment run–off.
Such basins may be “wet” (with permanent water), “dry” (filling with water only during storm events), or
“mixed” (functioning more like a wetland, and capable of sustaining “hydrophytic” vegetation characteristic
of water-logged soil). The terminology has been quite inconsistent, but wet basins are usually termed
“wet detention basins” (we refer to them here simply as stormwater ponds). Dry swales by contrast are often
termed “retention basins”. Examples of a dry retention basin and wet pond are shown in Figure 12a–b.
Although wet and dry basins differ, a major purpose of each is to allow the large amounts of sediment,
nutrients and toxins carried by rainwater to remain within the basin, thus protecting water quality
downstream. Design features are intended to slow down passage of the rainwater through the basin (increase
its hydraulic residence time). These features often include large length–to–width ratios (extending the
distance between inflow and outflow structures), the presence of baffles to prevent
water from heading directly from inflow to outflow, the positioning of the outflow high enough on the basin
slope to prevent water from smaller storm events from leaving the basin, and optimizing the cross-sectional
area of the outlet pipe to regulate flow.
Interestingly, although sediment retention is the primary goal of wet (storm water) ponds, there appears to
be little monitoring of basin pond condition by the townships or county, although increasing sediment
accumulation (and thus loss of water volume) over time gradually reduces basin effectiveness.
Aesthetics are rarely considered in constructing/managing stormwater ponds, which are perhaps as
distinguishable by the absence of their riparian amenities as by the presence of cement inflow and outflow
structures. The engineering firm Princeton Hydro has been a regional leader in attempting to reverse the
more traditional, single-minded view of storm water ponds as simply pollution control devices.